This invention relates to a method and apparatus for constrained distribution of television program material.
Referring to FIG. 1 of the drawings, a television program provider, such as a company that operates a broadcast network, may operate a production facility 6 at which it produces a program signal AV having a baseband video component representing a sequence of pictures and at least one corresponding baseband audio component. In order to distribute the program signal to a wide audience of viewers, the program provider, a satellite uplink operator and a cable network operator enter into commercial arrangements whereby the program provider provides program content to the uplink operator, which delivers the content to the cable operator, which in turn delivers the content to viewers having decoding and presentation equipment connected to a cable distribution network 10.
The commercial arrangements between the uplink operator and the cable operator may provide that the uplink operator will supply certain services (corresponding to what are commonly referred to as channels, such as NBC or FOX, in the broadcast television domain) to the cable operator. The uplink operator has an interest in ensuring that the program material delivered by the cable operator to viewers meets quality standards prescribed by the program provider.
The uplink operator receives the baseband program signal AV provided by the program provider (for example on tape or by accessing a server) and supplies the program signal to an encoder/multiplexer 12, which encodes the pictures using a video coding algorithm and thereby creates a bitstream that represents a corresponding sequence of coded pictures (also known as video access units). For the purpose of this description we shall assume that the video coding algorithm produces a bitstream that conforms to the video coding standard known as MPEG 4. The encoder/multiplexer also encodes the corresponding audio signal(s) and creates a bitstream representing a sequence of coded audio frames (also known as audio access units). The encoder/multiplexer 12 packetizes the bitstreams as video and audio packetized elementary streams (PESs), encrypts the video and audio PESs, and combines the encrypted video and audio PESs with video and audio PESs for other services offered by the program provider to form an MPEG multi-program transport stream (MPTS). A transmitter 14 employs the MPTS bitstream to modulate an RF carrier and transmits the modulated carrier via a satellite transponder (not shown) to a cable distribution system headend 20. Current transponder channels have sufficient capacity (up to 77 Mb/s) to transmit an MPTS containing ten or more high definition (HD) MPEG 4 services, each corresponding to a single broadcast television channel. The uplink operator may also operate other encoder/multiplexers and transmitters for creating and transmitting other MPTSs containing other program signals via respective transponder channels.
The headend 20 includes several receivers 24 that are tuned to the transmission frequencies of the transponders respectively and recover the MPTS bitstreams from the respective RF carriers, extract the encrypted MPEG 4 bitstreams from the MPTSs, and decrypt the MPEG 4 bitstreams.
MPEG 4 provides substantially better compression of video material than the video coding standard known as MPEG 2, but there is a large installed base of MPEG 2 set top decoders. Accordingly, although the uplink operator typically transmits video material in the form of MPEG 4 data, as discussed above, the cable distribution system operator is constrained by the needs of the installed base to supply the video material to the subscribers in the form of MPEG 2 data. Therefore, the headend 20 also includes transcoders 26 that transcode the MPEG 4 bitstreams to MPEG 2, and a multiplexer 30 that creates one or more MPTSs each containing several MPEG 2 services. Transmitters 34 utilize respective cable channels to transmit the MPEG 2 MPTSs to subscriber nodes over the cable network 10.
The decoding and presentation equipment at a subscriber node may include a set top decoder 38 and a television set 40. The set top decoder includes suitable devices for selecting a service based on a channel selection command provided by the subscriber, typically using a remote control unit, decomposing the cable channel MPTS that contains the selected service, and decoding the audio and video bitstreams for the selected service to create suitable audio and video signals for driving the television set 40.
Each cable distribution channel may be limited to a 38.8 Mb/s data rate. Different types of programming require different data rates to provide satisfactory viewing experiences. For example, an HD sports service generally requires a considerably higher bit rate (perhaps 17 Mb/s) than an HD movie service (15 Mb/s), which in turn requires a considerably higher bit rate than an SD sports service (3 Mb/s).
In order to optimize utilization of the capacity of the cable channel, the MPEG 2 services may be statistically multiplexed. Statistical multiplexing is commonly used to compose an MPTS having a maximum bit rate that is less than the sum of the peak bit rates of the individual program streams. Currently, the manner in which the statistical multiplexing process allocates bit rate to the MPEG 2 services is under control of the cable operator, and the manner in which the cable operator exercises this control might not always be in accordance with the wishes of the uplink operator or program provider. For example, a program provider might provide three services with the intention that one of these services should be presented with a higher quality even at the expense of one of the other services, and the cable operator might not always allocate bit rate in a manner that is consistent with this goal.